Inverse determination of building heating profiles from the knowledge of measurements within the turbulent slot-vented enclosure

Slot ventilated enclosure flows have been simulated, respectively in displacement ventilation and mixed ventilation covering from the forced convection dominated flow to the natural convection dominated flow. Direct convection simulation together with the turbulent streamlines and turbulent heatlines demonstrate that the enclosure flow pattern, indoor thermal level and heat transfer potential will depend on the interactions of external forced flow and thermal buoyancy driven flows, i.e., Reynolds number and Grashof number. In subsequent inverse convection modeling, the inverse determination of enclosure wall heat flux profiles was conducted by the use of adjoint methodology, in which the direct, sensitivity and adjoint problems are formulated and solved by finite volume method. The effects of the supplying air flow rate, thermal source strength, ventilation mode, flux functional forms, and the measurement errors on the accuracy of inverse turbulent convection estimation have been investigated. The inverse solutions of turbulent convections are of low level accuracy as the flow becomes thermal-driven turbulent flows, and they deteriorate as the noise levels increase. This work is of fundamental importance for the room air flow design and measurements involving the turbulent thermal convections.     

Thermal and Moisture Transport Inhibitions in a Moist Air Saturated Enclosure Attached with Protruding Partitions for Built Energy Conservations

ABSTRACT

Combined heat and moisture transportation in an enclosure has been numerically investigated, which could benefit the sustainable building energy conservations and electronic cooling designs. An adiabatic and impermeable partition of finite thickness is considered, placed in the enclosure following an ordered arrangement. Effects of length and location of the partition, buoyancy ratio and thermal Rayleigh number on convective heat and moisture transfer rates in the enclosure are discussed. Firstly, this situation of the partition placed in the horizontal wall is studied, where inhibition effect of partition is observed. It is seen that the location of partition put relatively weaker influences on the heat and mass transfer in the regime of thermal-driven flow, when its length exceeds the critical value. Additionally, inhibition effect is more pronounced as the partition is fixed in center of vertical wall. Furthermore, local heat and mass transfer rates could be suppressed when the buoyancy ratio becomes negative. Finally, thermal Rayleigh number greatly affects the transport structures of fluid, heat and moisture, whatever aiding flow or opposing flow situations. Heat and mass transfer potentials could be promoted with increasing thermal Rayleigh numbers. Present work could be adopted to optimize the enclosure flows simultaneously with heat and moisture transport.     

Hydrogen non-premixed combustion in enclosure with one vent and sustained release: Numerical experiments

Numerical experiments are performed to understand different regimes of hydrogen non-premixed combustion in an enclosure with passive ventilation through one horizontal or vertical vent located at the top of a wall. The Reynolds averaged Navier–Stokes (RANS) computational fluid dynamics (CFD) model with a reduced chemical reaction mechanism is described in detail. The model is based on the renormalization group (RNG) k-ε turbulence model, the eddy dissipation concept (EDC) model for simulation of combustion coupled with the 18-step reduced chemical mechanism (8 species), and the in-situ adaptive tabulation (ISAT) algorithm that accelerates the reacting flow calculations by two to three orders of magnitude. The analysis of temperature and species (hydroxyl, hydrogen, oxygen, water) concentrations in time, as well as the velocity through the vent, shed a light on regimes and dynamics of indoor hydrogen fires. A well-ventilated fire is simulated in the enclosure at a lower release flow rate and complete combustion of hydrogen within the enclosure. Fire becomes under-ventilated at higher release flow rates with two different modes observed. The first mode is the external flame stabilised at the enclosure vent at moderate release rates, and the second mode is the self-extinction of combustion inside and outside the enclosure at higher hydrogen release rates. The simulations demonstrated a complex reacting flow dynamics in the enclosure that leads to formation of the external flame or the self-extinction. The air intake into the enclosure at later stages of the process through the whole vent area is a characteristic feature of the self-extinction regime. This air intake is due to faster cooling of hot combustion products by sustained colder hydrogen leak compared to the generation of hot products by the ceasing chemical reactions inside the enclosure and hydrogen supply. In general, an increase of hydrogen sustained release flow rate will change fire regime from the well-ventilated combustion within the enclosure, through the external flame stabilised at the vent, and finally to the self-extinction of combustion throughout the domain.     

Transient natural convective heat transfer of a low-Prandtl-number fluid from a heated horizontal circular cylinder to its coaxial triangular enclosure

Transient natural convective heat transfer of liquid gallium, which has a Prandtl number of 0.023 at 310 K, from a heated horizontal circular cylinder to its coaxial triangular enclosure is studied numerically by employing the control volume method. Two orientations of the triangular cylinder are investigated and the Grashof number is varied from 10⁴ to 10⁷. Development of natural convection is presented by means of the evolutions of the average Nusselt number over the outer triangular wall. Temporal stages during the course of development are identified and demonstrated through representative snapshots of streamlines and isotherms. The time-averaged Nusselt number is scaled with Grashof number for both conduction- and convection-dominated regimes. It is found that by placing horizontally the top side of the triangular cylinder, the convective flow becomes more stable and the overall heat transfer is enhanced. In addition, pitchfork bifurcation is explored quantitatively and its onset times are predicted as well.     

Unsteady natural convection heat transfer from a heated horizontal circular cylinder to its air-filled coaxial triangular enclosure

Unsteady natural convection heat transfer in a horizontal annular region bounded by a heated inner circular cylinder and a coaxial outer triangular cylinder is numerically studied for a wide range of Grashof numbers, aspect ratios, and inclination angles of the triangular enclosure. Different phases are identified during the course of flow development through the evolutions of the average Nusselt number over the inner circular wall. Snapshots of streamlines and isotherms for two typical cases are presented to exhibit identification among these phases. The flow development time and time-averaged Nusselt number over the inner circular wall are predicted and scaled with Grashof number. Additionally, the onset and evolution of pitchfork bifurcation are quantitatively investigated.     

Laminar heat transfer in an asymmetrically heated rectangular duct

This paper presents a numerical study concerning the effects of non-uniform heating on the heat transfer of a thermally undeveloped gas flow in a horizontal rectangular duct; a vertical side wall is uniformly heated, and the other walls are insulated. As an initial step of the study, the duct flow is assumed to be laminar, and buoyancy effects are considered. The heat transfer rate and drag increase with the secondary flow due to buoyancy; the effects of the buoyancy force on the heat transfer and friction coefficient of the thermally undeveloped region are found to depend only upon modified Grashof numbers of the duct entrance.     

Double diffusion, natural convection in an enclosure filled with saturated porous medium subjected to cross gradients; stably stratified fluid

Two- and three-dimensional flows, heat and mass transfer in a horizontal enclosure with aspect ratio of two filled with saturated porous medium were analyzed numerically. The enclosure is heated differentially and stably stratified species concentration is imposed vertically. The Prandtl number is fixed to 10 (aqueous solutions). The Lewis number is varied in the range of 1.0-1000 to cover a wide range of species diffusion material in water. The work is concentrated on stable stratified flow. The results of two- and three-dimensional models were compared. Interesting results are obtained for a wide range of solutal to thermal buoyancy ratios. The difference in the rate of heat and mass transfer between prediction of three- and two-dimensional simulations is not that significant, even though the flow exhibits a three-dimensional structure. This is due to the fact that the spanwise flow is very weak when compared with main flow.     

Effect of corrugation frequencies on natural convective heat transfer and flow characteristics in a square enclosure of vee-corrugated vertical walls

A numerical investigation has been carried out on natural convective heat transfer and fluid flow in a square cavity with vee-corrugated vertical surfaces. This study covers the range of corrugation frequency from 1 to 3 and Grashof number from 10³ to 10⁵. The corrugation amplitude has been fixed at 5% of the enclosure height. The vorticity stream function formulation with the control volume based finite element method has been used to analyse the effects of corrugation frequency and Grashof number. The investigation shows that the overall heat transfer through the enclosure increases with the increase of corrugation for low Grashof number; but the trend is reversed for high Grashof number.     

Unsteady natural convection in a water-filled isosceles triangular enclosure heated from below

This study is concerned with transient natural convection in a water-filled isosceles triangular enclosure subject to cooling at the inclined surfaces and simultaneous heating at the base. The unsteady flows over a range of Grashof numbers are visualized using a shadowgraph technique, and corresponding numerical simulations are carried out using a Finite Volume Method. Both the experiments and numerical simulations have revealed that the transient flow development in the enclosure due to abrupt heating and cooling through the boundaries can be classified into three distinct stages, that is, an early stage, a transitional stage, and a steady/quasi-steady stage. The major flow features at each of the three stages are described and the Grashof number effects on the flow development and heat transfer are discussed. It is found that, for a fixed aspect ratio of 0.5, a transition of the unsteady flow from symmetric to asymmetric structures occurs for Grashof numbers above 2.95 × 10⁴. Moreover, the present heat transfer calculations indicate that the average Nusselt number over the inclined and horizontal surfaces approximately scales with Gr^0.2.     

Stability of buoyancy and surface tension driven convection in a horizontal double-diffusive fluid layer

Linear stability analysis has been applied to examine the stability of convection in a horizontal double-diffusive fluid layer driven by the combined effects of buoyancy and surface tension. Such a convective flow may serve as an idealized model of the horizontal Bridgman process for crystallization or solidification of liquid melts. Results show that salt-finger instability is excited over a wide range of thermal and solutal Grashof numbers. Travelling wave instabilities caused by surface tension effects are excited when the effective Marangoni number becomes large.     

Natural Convection in Power-Law Fluids in a Square Enclosure from Two Differentially Heated Horizontal Cylinders

Laminar natural convection has been numerically investigated from two differentially heated horizontal cylinders in a square enclosure filled with power-law fluids. Two basic configurations, namely, vertical- and diagonal-alignment of the cylinders at various locations have been considered. The coupled continuity, momentum and energy equations have been solved numerically to elucidate the effect of the Grashof number (10²–10⁴), Prandtl number (0.7–100) and power-law index (0.2–2) for a range of symmetric and asymmetric locations of the cylinders. The velocity and temperature fields are visualized in terms of streamlines, isothermal contours and plots of the local and average Nusselt number for different positions of the cylinders. The occurrence of the power-law index in the definitions of the Grashof and Prandtl numbers accentuates the interplay between the viscous, inertial and buoyancy forces thereby leading to nonlinearity in the observed trends. The presence of the dead zones coupled with the dominance of conduction under certain conditions strongly influences the overall heat transfer. All else being equal, it is possible to improve heat transfer for asymmetric positioning of the cylinders, especially at high values of the Prandtl number and Grashof number in shear-thinning fluids. A predictive correlation has been developed thereby enabling the estimation of the heat transfer coefficient in a new application in terms of the geometric and kinematic parameters.     

Effect of Prandtl Number on Free Convection from Two Cylinders in a Square Enclosure

This study explores the effect of Prandtl number on the laminar natural convection heat transfer to Newtonian fluids in a square enclosure consisting of one hot circular cylinder and one cold circular cylinder. The walls of the square enclosure are maintained isothermal and at the same temperature as the cold cylinder and the fluid medium. The governing partial differential equations have been solved numerically over the following ranges of conditions: Grashof number, 10 to 10⁵; Prandtl number, 0.7 to 100 (or the range of Rayleigh numbers as 7 to 10⁷); and relative positioning of the cylinders, ?0.25 to 0.25. However, the ratio of the radius of the cylinder to the side of the enclosure is held fixed at 0.2. Extensive results on the streamline and isotherm contours, the local Nusselt number distribution, and the average Nusselt number are discussed to delineate the influence of Grashof and Prandtl numbers on them for a given location with respect to the horizontal center line. The surface-averaged Nusselt number shows a positive dependence on Grashof and Prandtl numbers for a fixed location of the two cylinders. The heat transfer results have been correlated as a function of the Rayleigh number and geometric parameters, thereby enabling its prediction in a new application.     

Free Convection in Confined Power-Law Fluids From Two Side-by-Side Cylinders in a Square Enclosure

Laminar free convection heat transfer in power-law fluids from two side-by-side cylinders (one hot and one cold) confined in a square duct has been studied numerically in the two-dimensional flow regime. For a fixed value of the ratio of cylinder radius to size of the enclosure, the effect of geometrical placement of the cylinders is studied on the resulting velocity and temperature fields in the laminar free convection regime by considering six asymmetric locations of the two cylinders. In particular, extensive results reported herein span the range of conditions of Grashof number, 10 to 10⁵; Prandtl number, 0.7 to 100, thereby yielding the range of the Rayleigh number as 7 to 10⁷; power-law index, 0.3 to 1.8; and the relative positions (dimensionless) of the cylinders with respect to the centerline, –0.25 to 0.25. The heat transfer characteristics are analyzed in terms of the local Nusselt number along the surfaces of the two cylinders and the enclosure walls. Overall, the average Nusselt number shows a positive dependence on both the Grashof number and the Prandtl number irrespective of the values of power-law index and relative positioning of the cylinders. Also, all else being equal, shear-thinning fluid behavior promotes heat transfer with reference to that in Newtonian fluids. When the two cylinders are situated close to the bottom wall, the rate of heat transfer is augmented with reference to that for the symmetric positioning of the cylinders along the horizontal mid-plane of the enclosure. Conversely, heat transfer deteriorates as the cylinders are located above the centerline of the enclosure. The present numerical results have been consolidated via the use of a modified Rayleigh number, thereby enabling the estimation of the average Nusselt number in a new application.     

DOUBLE-DIFFUSIVE CONVECTION IN A POROUS ENCLOSURE WITH COOPERATING TEMPERATURE AND CONCENTRATION GRADIENTS AND HEAT GENERATION OR ABSORPTION EFFECTS

The problem of unsteady, laminar double-diffusive convective flow of a binary gas mixture in a rectangular enclosure filled with a uniform porous medium is considered. A temperature-dependent heat source or sink is assumed to exist within the enclosure boundaries. Transverse cooperating gradients of heat and mass are applied on the two opposing vertical walls of the enclosure while the other two horizontal walls are adiabatic and impermeable to mass transfer. A numerical solution based on the finite-difference methodology is obtained. Representative results illustrating the effects of the inverse Darcy number, the heat generation or absorption coefficient, and the buoyancy ratio on the contour maps of the streamline, temperature, and concentration as well as the profiles of velocity, temperature, and concentration at the midsection of the enclosure are reported. In addition, results for the average Nusselt and Sherwood numbers are presented in tabulated form and discussed for various parametric conditions.     

Developing laminar mixed convection with heat and mass transfer in horizontal and vertical tubes

The effects of the solutal and thermal Grashof numbers on the flow, temperature and concentration fields in tubes with uniform heat flux and concentration at the fluid-solid interface have been investigated numerically using a three-dimensional axially parabolic model. Results show a complex development of the flow field which is strongly influenced by the values of the two Grashof numbers and by the tube inclination. For vertical tubes the flow field is also influenced by the relative direction of the flow and the buoyancy forces. In general, very close to the tube inlet forced convection boundary layer development dominates. Further downstream, the effects of solutal buoyancy predominate while those of thermal origin determine the flow field far downstream and, in particular, the fully developed conditions. The axial evolution of the wall shear stress τ_z, the Nusselt number Nu_z and the Sherwood number Sh_z in both horizontal and vertical tubes are presented for different combinations of the two Grashof numbers. For horizontal tubes and vertical tubes with upward flow these three variables are greater than the corresponding ones for forced convection. The opposite is true for downward flow in vertical tubes.     

Oscillatory mixed convection in the jet impingement cooling of a horizontal surface immersed in a nanofluid-saturated porous medium

Oscillatory mixed convection in the jet impingement cooling of a partially heated horizontal surface immersed in a nanofluid-saturated porous medium is simulated and discussed in this study. This situation appears when the jet flow and the flow due to buoyancy have opposing effects and are in conflict for domination. The aim of the present contribution is to explore how governing parameters may alter these oscillations and the resulting heat exchange. It is demonstrated that the final steady or oscillatory flow response depends on the values of the Reynolds number, the Grashof number, and the Darcy number but not influenced by the medium porosity and the nanoparticles fraction.     

Effect of wind and buoyancy on hydrogen release and dispersion in a compartment with vents at multiple levels

The natural and forced mixing and dispersion of hydrogen released in an accidental manner in a partially enclosed compartment with vents at multiple heights is investigated using theoretical tools. The key to the analysis is determination of the position of the neutral buoyancy plane, where the pressure in the compartment is equal to that of the exterior. Air flows in through vents below the position of neutral buoyancy and exits from vents above it. CFD simulations are conducted to confirm the physical phenomena and to compare with the analytical results. The analytical model is useful in understanding the important physical processes involved during hydrogen release and dispersion in a compartment with vents at multiple levels, with and without a steady wind. Parametric studies are conducted to identify the relative importance of various parameters. Model results indicate that the steady-state hydrogen volume fraction in the compartment is lower when the hydrogen release rate is smaller and the vent cross-sectional area is larger. Results also indicate that the fastest way to reduce flammable levels of hydrogen concentration in a compartment can be accomplished by blowing through the vents.     

Mixed convection in a square vented enclosure filled with a porous medium

Steady mixed convection flow in a vented enclosure with an isothermal vertical wall and filled with a fluid-saturated porous medium is investigated numerically. The forced flow conditions are imposed by providing an inlet at the bottom surface, and a vent at the top, facing the inlet. The nature and the basic characteristics of the mixed aiding as well as mixed opposing flows that arise are investigated using the Darcy law model. The governing parameters are the Rayleigh number, Péclet number, and the width of the inlet as a fraction of the height of the square enclosure. These parameters are varied over wide ranges and their effect on the heat transfer characteristics is studied in detail.     

Interaction of Participating Medium Radiation with Thermosolutal Convection—A Critical Appraisal

The present study addresses the interaction effect of participating media radiation with the onset of double‐diffusive convection in a square enclosure. Vertical walls are imposed with constant temperature and concentration, and the horizontal walls are impermeable and adiabatic. The boundaries of the enclosure are diffuse‐gray, and the enclosed fluid isotropically scatters, emits, and absorbs thermal radiation. Numerical simulations have been performed for both aiding and opposing buoyancy conditions. The buoyancy ratio has been varied to simulate the effect of buoyancy driven flow and compositionally driven flow, along with transition of flow between the above. Optical properties like opacity of medium, scattering albedo, Planck number, and wall emissivity have been varied to depict their influence on flow and heat transfer. The modified MAC method is used for the solution of convective transport equations. Gradient dependent consistent hybrid upwind scheme of second order (GDCHUSSO) is used for the discretization of the convective terms. The discrete ordinate method, with S8 approximation, is used to solve the radiative transport equation. The parametric results are provided in graphical and tabular form. Flow lines, isotherms, and isoconcentration contour maps are provided to bring clarity in the understanding of the momentum, heat, and solute transport phenomenon. The stabilization effect of thermal radiation at critical buoyancy ratio for buoyancy opposed flow is observed. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21108     

Combined heat and moisture convective transport in a partial enclosure with multiple free ports

Combined heat and moisture transport in an enclosure with free ports has been investigated numerically. Enclosed moist air interacts with the surrounding air through these free-vented ports. The governing conservation equations were solved numerically using a control volume-based finite difference technique. Appropriate velocity boundary conditions at each ports are imposed to achieve overall mass conservation across this system. Air, heat and moisture transport structures are visualized respectively by streamlines, heatlines and masslines. Effects of buoyancy ratio, thermal Rayleigh number on convective heat/moisture transfer rate and flow rate across each free-vented port are discussed. Particularly, Numerical results demonstrate that the convective heat and moisture transport patterns and transport rates on horizontal ports greatly depend on properties of porous medium, while the air exchange rate on vertical port is almost unaffected by the buoyancy ratios for most situations.